Natural pears aren't terribly rare. Natural pearls pretty enough to wear are another story entirely... So, in the last bit of the nineteenth century, a few fellows were working on convincing pearl oysters to make greater quantities of salable pearls. The most notable of these was Kokichi Mikimoto.
Cultured pearls are less expensive, more predictable in shape, size, and color than natural pearls. The short story goes like this: human opens oyster, inserts irritating object, closes the oyster and puts it back in the water. Come back a year or three later and there's a cultured pearl in there! The shape and size of the nucleus informs the shape and size of the end product.
Mikimoto's first pearl culturing success came in 1893 with hemispherical Mabe pearls. Twelve years later, he began producing spherical pearls.
He used the saltwater akoya oyster, or the pinctada fuctada, to make his pearls. This oyster produces goods ranging from 2mm to 10mm in diameter. Typical sizes are 6 to 7mm. The excellent consistency in shape and size makes them the perfect choice for matched pearl strands.
Akoya pearls are produced in Japan, China, and Vietnam. The akoya oyster is nucleated at the age of 2 to 3 years and the pearl allowed 6 months to 2 years to form. They accept anywhere from 1 to 5 nuclei and can only be nucleated once. They produce an unusually high percentage of spherical pearls: 70%